The present invention relates to image processing and more particularly to a spatial thresholding method for thresholding the luminance component of color pels in a discrete color video image, prior to compression.
Because travel costs are rising and because a traveler's time in transit can seldom be used productively, there is an increasing interest in the use of video conferencing as an alternative to face to face business meetings between people from different locations. In a typical video conferencing system, people in different cities or even different countries meet in special video conferencing rooms at their respective home locations. Each room normally includes a room camera for capturing a wide angle view of the people, a document camera which can be focussed on letters, drawings or other documents, a room monitor for permitting people in one room to see those in the other, and a document monitor for viewing documents being presented in the other room. Each room also includes one or more microphones and speakers. Audio/video communications between the two rooms are established over conventional telecommunication links, such as leased or switched telephone lines or satellite communication channels.
The simplest form of video conferencing system is one which uses conventional video equipment and data transmission techniques to provide what is referred to as full-motion video conferencing; that is, video conferencing in which the people in one room can watch those in the other room move about during the teleconference. The communication costs for conventional full-motion video conferencing are high. A considerable amount of data must be transmitted at high rates, making it necessary to use a transmission medium having a high bandwidth. Communications costs are generally proportional to bandwidth. Therefore, any requirement for a high bandwidth runs counter to one of the primary reasons for using video conferencing to begin with, namely, to reduce costs associated with the conduct of meetings.
To reduce communications costs, freeze-frame video conferencing techniques may be employed. The video image captured by a room camera is updated only periodically, either at fixed intervals or on command of an operator. People at a receiving location see the same "frozen" room image between updates. Audio or voice signals are transmitted on a real time basis so that there are no perceptible delays in voice communications. Document images are updated only when the person presenting a document pushes a "send" button in the video conferencing room.
There are two basic ways to reduce bandwidth requirements in a freeze-frame video conferencing system. One way is to reduce the amount of data that must be sent in order to recreate an acceptable image at the receiving location. The other way is to use a lower bandwidth and simply take longer to transmit the data required to reconstruct an acceptable video image at the receiving location.
The time required to transmit necessary image data is important in any freeze-frame video conferencing system since it determines the frequency with which images can be updated during the course of a teleconference. If meeting participants must sit and wait for an expected video image, those participants are likely to become irritated and restless, reducing the effectiveness of the teleconference.
In a freeze-frame video conferencing system designed to capture, transmit and display only monochrome images, it is well known to reduce transmission time requirements by compressing the amount of data to be transmitted. There are two basic types of data compression techniques which are used for monochrome video data. The first type of technique is used most often in facsimile systems designed for transmitting document images. Facsimile data compression techniques assume that all picture elements or pels in a video image are intended to be either black or white. When a video image is captured by a video camera, every pel in that image is assigned either a black value or a white value depending upon whether the gray value of the pel is greater than or less than a threshold value. A document thresholded in this manner includes long runs of black or white pels. Known run length encoding techniques can be used to reduce the amount of data which must be transmitted in order to represent those runs of pels.
The second major type of monochrome data compression technique is one which is used with continuous gray scale images; that is, images having many different shades of gray. One known continuous gray scale technique requires that the value of each pel in the image be predicted using the known values of one or more pels in the preceding scan line and one or more preceding pels in the same scan line. The actual value of the subject pel is compared to its predicted value. The difference between the predicted and actual values is transmitted.
There as been a good deal of interest in the use of color video in video conferencing systems because information presented in the form of a color image is generally considered easier to comprehend than the same information presented in the form of a monochrome or gray scale image. However, the use of color video is not without problems. More data is required to define a pel in a color image than a pel in a monochrome image. A color includes both luminance (brightness) and chrominance (color}components while a monochrome pel includes only a luminance component. Techniques used successfully in monochrome videoconferencing systems are not necessarily suitable for color video conferencing systems.
When each pel in a color image is broken down or decomposed into its luminance and chrominance components, the continuous gray scale technique described above can be used on continuous color images by independently compressing the luminance and chrominance components. This technique works well for continuous color images in which the luminance and chrominance of pels vary continuously. Images of people and objects fall into this category. However, the gray scale technique does not produce optimum compression for discrete color images in which the luminance and chrominance of pels remain nominally constant throughout relatively large, well defined areas. Also, facsimile techniques are completely unsuitable since such techniques would assign either black or white value to the luminance of each pel. A new technique is needed to compress the luminance component of discrete color images.